1. 1 Understanding Buffer Size Requirements in a Router Thanks to Nick McKeown and John Lockwood for numerous slides

2. 2 Buffer Requirements in a Router Buffer size matters: –Small queues reduce delay –Large buffers are expensive Theoretical tools predict requirements –Queuing theory –Large deviation theory –Mean field theory Yet, there is no direct answer –Flows have a closed-loop nature –Question arises on whether focus should be on equilibrium state or transient state

3. 3 Universally applied rule-of-thumb: –A router needs a buffer size: –2T is the two-way propagation delay (or just 250ms) –C is capacity of bottleneck link Context –Mandated in backbone and edge routers –Appears in RFPs and IETF architectural guidelines –Already known by inventors of TCP [Van Jacobson, 1988] –Has major consequences for router design Rule-of-thumb C Router Source Destination 2T

4. 4 The Story So Far 10,000 20 # packets at 10Gb/s 1,000,000 (1)Assume: Large number of desynchronized flows; 100% utilization (2)Assume: Large number of desynchronized flows; <100% utilization

5. 5 Using NetFPGA to explore buffer size Need to reduce buffer size and measure occupancy Alas, not possible in commercial routers So, we will use the NetFPGA instead Objective: –Use the NetFPGA to understand how large a buffer we need for a single TCP flow.

6. 6 Rule for adjusting W –If an ACK is received:W ← W+1/W –If a packet is lost:W ← W/2 Why 2TxC for a single TCP Flow? Only W packets may be outstanding

7. 7 Time evolution of a single TCP flow through a router. Buffer is < 2T*C Time Evolution of a Single TCP Flow Time evolution of a single TCP flow through a router. Buffer is 2T*C

8. 8 Instructions for a real evaluation using NetFPGA Observing and Controlling the Queue Size

9. 9 Setup for the Buffer Evaluation Local Host NetFPGA Adjacent Web & Video Server eth1 eth2 nf2c2 nf2c1 Router

10. 10 Enhanced Router Objectives –Observe router with new modules –New modules: rate limiting, event capture Execution –Run event capture router –Look at routing tables –Explore details pane –Start tcp transfer, look at queue occupancy –Change rate, look at queue occupancy

11. 11 Step 1 - Run Pre-made Enhanced Router Start terminal and cd to “NF2/projects/tutorial_ro uter/sw/” Type “./tut_adv_router_gui.pl” A familiar GUI should start

12. 12 Step 2 - Explore Enhanced Router Click on the Details tab A similar pipeline to the one seen previously shown with some additions

13. 13 Enhanced Router Pipeline Two modules added 1.Event Capture to capture output queue events (writes, reads, drops) 2.Rate Limiter to create a bottleneck MAC RxQ MAC RxQ CPU RxQ CPU RxQ MAC RxQ MAC RxQ CPU RxQ CPU RxQ MAC RxQ MAC RxQ CPU RxQ CPU RxQ MAC RxQ MAC RxQ CPU RxQ CPU RxQ Input Arbiter Output Port Lookup MAC TxQ MAC TxQ CPU TxQ CPU TxQ MAC TxQ MAC TxQ CPU TxQ CPU TxQ MAC TxQ MAC TxQ CPU TxQ CPU TxQ MAC TxQ MAC TxQ CPU TxQ CPU TxQ Output Queues Rate Limiter Rate Limiter Event Capture

14. 14 Step 3 - Decrease the Link Rate To create bottleneck and show the TCP “sawtooth,” link-rate is decreased. In the Details tab, click the “Rate Limit” module Check Enabled Set link rate to 1.953Mbps

15. 15 Step 4 – Decrease Queue Size Go back to the Details panel and click on “Output Queues” Select the “Output Queue 2” tab Change the output queue size in packets slider to 16

16. 16 Step 5 - Start Event Capture Click on the Event Capture module under the Details tab This should start the configuration page

17. 17 Step 6 - Configure Event Capture Check Send to local host to receive events on the local host Check Monitor Queue 2 to monitor output queue of MAC port1 Check Enable Capture to start event capture

18. 18 Step 7 - Start TCP Transfer We will use iperf to run a large TCP transfer and look at queue evolution Start a terminal and cd to “NF2/projects/tutorial_router/sw” Type “./iperf.sh”

19. 19 Step 8 - Look at Event Capture Results Click on the Event Capture module under the Details tab. The sawtooth pattern should now be visible.

20. 20 Queue Occupancy Charts Leave the control windows open Observe the TCP/IP sawtooth